Fuel cell power plants are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus. In such power plants, one or typically a plurality, of planar fuel cells are arranged in a fuel cell stack, or cell stack assembly (CSA). Each cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. The reducing fluid and the oxidant are typically delivered to and removed from the cell stack via respective manifolds. In a cell using a proton exchange membrane (PEM) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes, depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is a PEM electrolyte, which consists of a solid polymer well known in the art. Other common electrolytes used in fuel cells include phosphoric acid, sulfuric acid, or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials, is fixed and cannot be leached from the cell, and has a relatively stable capacity for water retention. Typically also, provision is made for a coolant system in association with the fuel cell for removing product water from the cell, for serving a cooling function, and for providing a source of water for other functions associated with the fuel cell power plant. There should be a general balance of water in the coolant system for the reasons mentioned above and for the specific uses to be described. The power plant should be self sufficient with respect to water consumption in order to avoid, or at least minimize, dealing with possible impurities in make-up water.
It is known to recycle both heat and water contained in various power plant exhaust gases to improve the efficiency of the system and maintain the water balance. Typically, this may be done by passing the incoming air for the cathode through a sink channel of an energy transfer device (ERD), sometimes also referred to as a water transfer device (WTD), and passing the warm and moisture-laden exhaust gases through an adjacent source channel of the device. An enthalpy exchange barrier separates the source and sink channels and allows the transfer of heat and water vapor from the exhaust gases flowing in the source channel to the air flowing in the sink channel. This serves to humidify the incoming air supplied to the cathode, and does so in a manner that retains water within the system. One source of moisture-laden exhaust gas is the exhaust stream from the cathode itself. An example of such an arrangement is disclosed in U.S. Pat. No. 6,120,923 to Leslie L. Van Dine, et al, assigned to the assignee of the present invention. Another similar arrangement is described in U.S. Pat. No. 6,274,259 to Albert P. Grasso, et al, also assigned to the assignee of the present invention.
In addition to the fuel cell stack assembly, many fuel cell power plants also include the capability of processing a source of raw fuel into a hydrogen-rich fuel stream as the reducing fluid for delivery to the anode of the CSA. The raw fuel is typically some form of hydrocarbon, and a fuel processing system (FPS) is used to reform the fuel to the desired hydrogen-rich stream. A typical FPS uses one or more reactors to reform the raw hydrocarbon to a hydrogen-rich stream having acceptably low levels of CO. In almost all such instances, the FPS includes reaction means, such as a catalytic steam reformer (CSR), an autothermal reformer (ATR), or a catalytic partial oxidizer (CPO), to effect the basic reformation of the raw hydrocarbon fuel to a hydrogen-rich stream, and additional components may then enhance and clean that stream for use by the CSA. In each of those instances, the reaction means has a combustion process associated with it for the generation of heat, such that it may be termed a combustion-supported reaction means. The heat facilitates the reformation reaction and may be responsible for raising steam in certain instances.
The combustion that provides the heat may occur directly in the reactor, in a combustion or burner, zone, as in an ATR or a CPO, or it may take place in a burner externally of the actual reactor and be applied thereto through a heat transfer mechanism, as in a CSR. In either event, the same inlet air that is typically passed through the sink channel of the ERD for supplying oxidant to the cathode of the CSA may also be supplied to the combustion-supported reaction means to support at least the combustion process. Exhaust from the combustion-supported reaction means may be combined with the cathode exhaust to provide the exhaust gas stream that flows through the source channel of the ERD. This process is also described in the aforementioned U.S. Pat. Nos. 6,120,923 and 6,274,259.
While the humidification of the inlet air is generally beneficial for normal operation of the fuel cell power plant, and particularly the CSA, it is possible for that humidification and/or the mechanisms which provide it to experience extremes that are undesirable. For example, during start-up, the increased water content of the inlet air may overwhelm and prevent operation of the combustion-supported reaction means associated with the FPS. More particularly, for a range of combined oxygen, inert gas, and particular fuel gas concentrations, there exists a corresponding range of flammability for the combined gases. However, there also exists, for the range of combined gas concentrations, the flammability of the fuel gas as a function of the dew point of the resulting gas mixture. While the range or area of flammability for the combined gases may be relatively large, it is nevertheless critical that the dew point or water content of the air/fuel gas combination be such that a plot of the flammability as a function of dew point, falls within the overall range of flammability for proper combustion to occur. However, it is possible for the water content of the air issuing from the primary ERD to become sufficiently high during plant start-up that operation of the combustion-supported reaction means is not possible. This may occur because the cathode exhaust flow is negligible at that time, which means the controlling flow is the burner exhaust, which is much higher in temperature.
Thus, there is need for a technique of and system for, moisture stabilization of the inlet air supplied at least to the combustion-supported reaction means of a FPS in a fuel cell power plant. There is additionally need of attaining such moisture stabilization of the inlet air supplied to the combustion-supported reaction means in a manner and/or by means that is economical of space and/or weight.